The proposed investigation is a study of the development of thymocytes and their progeny (peripheral T cells). Of particular interest are the development of antigen-specific receptors, antigen responsiveness and effector functions. Clonal growth of thymocyte precursors, thymocytes and peripheral, antigen-responsive cells in tissue culture will be attempted in soft agar media supplemented with various growth factors. The progeny of these clones or cells isolated at various stages of their development from intact mice will be tested for their ability (1) to respond to putative thymic hormones and LAF, (2) to repopulate the T-dependent lymphoid tissues of irradiated animals, (3) to respond to allo and/or conventional antigens, and (4) to exert helper and/or suppessive effects on antigen-primed B cells. We further plan to immunoselect variants of cultured thymocytes and T cells which lack various cell surface markers and to examine the effects of these losses on the biological properties of the cells. Finally, we expect to produce a series of photoaffinity labels which should enable us to identify molecules which contribute to the antigen binding site of antigen-specific T lymphocytes.